The successful duplication and segregation of chromosome are essential for the maintenance of genomic integrity. Biochemical pathways, called checkpoints, provide the cell with a mechanism to sense DNA damage, and respond by arresting the cell cycle to allow DNA repair. The inability to respond by arresting the cell cycle to allow DNA repair. The inability to respond to such damage leads to increased genomic instability, which can contribute to deregulation of cellular growth and cancer Mutations in mammalian genes, such as p53 and ATM (Ataxia telangiectasia mutated), which abrogate this response, cause a genetic predisposition to cancer. At the cellular level, ATM-defective cells control are conserved among eukaryotes. In Schizosaccharomyces pombe (S. pombe), the Atm-like protein rad3 functions upstream of chk1 (checkpoint kinase 1). Chk1 function in S. pombe is required for arrest and survival following exposure to DNA-damaging agents such as ionizing radiation. We have isolated the budding yeast (Saccharomyces cerevisiae), human and murine homologues of the chk1 gene. Chk1 is required to regulate mitotic progression in response to DNA damage. The work described here will examine aspects of the DNA damage response pertaining to the Chk1 pathway that remain unanswered: 1) What is/are the pathway(s) leading to Chk1 activation? 2) Does the Chk1 pathway have a regulatory role in damage-induced DNA repair? 3) Are the transducers of the checkpoint signal associated with complexes involved in DNA replication and/or repair? 4) What are other effectors of the Chk1 pathway? Based on the conservation of CHK1 between the fission yeast and mammals, we predict that a member of the ATM family (Atm and/or related Atr) will regulate hk1 in response to DNA damage. We will undertake biochemical studies on proteins that associate with mammalian Chk1 to further elucidate the circuitry of the Chk1 pathway. Exploiting the evolutionary conservation of checkpoint regulatory components, we will identify the budding yeast homologues of hChk1 interactors and use the yeast model as a genetic tool to examine their roles in the cellular response to DNA damage. The primary goal of this work is to expand our understanding of how cells detect and respond to DNA damage. These studies will shed light on the mechanisms of the DNA damage response in mammalian cells, which may allow us to design more effective therapeutic regimens for the treatment of diseases that result from a deficiency in this capacity.